Problem: The scale on a map is 1cm : 5km. If the distance between two cities is 40km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 1cm on the map corresponds to an actual distance of 5km. An actual distance of 40km is the same as 8 $\cdot$ 5km. The distance between the two cities on the map is 8 $\cdot$ 1cm, or 8cm.